In recent years, a system for preventing vehicles from bumping into each other and protecting pedestrians from vehicles by using vehicle roadside communication and inter-vehicle communication has been studied. For example, Patent Document 1 and Patent Document 2 disclose a technique for providing a collision prevention system premised on use of the vehicle roadside communication. Further, Patent Document 3 discloses a technique for avoiding collision between vehicles by notifying positional information of a vehicle from an in-vehicle radio apparatus to another vehicle.                Patent Document 1: JP-2000-339591A        Patent Document 2: JP-2001-126193A (U.S. Pat. No. 6,337,637 B1)        Patent Document 3: JP-2002-260194A        
However, in the above-described conventional techniques, when in-vehicle radio apparatuses in multiple vehicles radio-transmit vehicle information simultaneously, the transmission waves interfere with each other, which disturbs information transmission to other vehicles.